|Title:||Nanoscale precipitation and mechanical properties of coherent precipitation–strengthened high-entropy alloys|
|Advisors:||Jiao, Zengbao (ME)|
Shi, Sanqiang (ME)
Hong Kong Polytechnic University -- Dissertations
|Department:||Department of Mechanical Engineering|
|Pages:||x, 122 pages : color illustrations|
|Abstract:||High-entropy alloys (HEAs) have drawn considerable attention as candidates for next-generation structural materials for a wide range of industrial applications due to their excellent mechanical properties. However, these solid-solution alloys are relatively weak in strength, which is far from the requirements for practical structural applications. Hence, the emphasis of HEA metallurgy has been placed on the strengthening of HEAs through various strengthening mechanisms. In particular, the precipitation of coherent L12 precipitates has been proved to be a powerful method for strengthening face-centered cubic (FCC) HEAs without causing a significant loss of ductility. To date, however, the atomistic understanding of precipitation mechanisms and the relationship between precipitate microstructure and mechanical properties of L12-strenthened FCC-base HEAs remain unclear. The objective of this thesis is to obtain a fundamental understanding of the mechanisms governing the formation and evolution of L12 precipitates and to correlate the precipitate microstructure with mechanical properties in the FCC base HEAs. First, this research explored the phase relationship, microstructure, and precipitation mechanisms of Fe-Co-Ni-Cr-Al-Ti-Nb alloys containing various types of intermetallic precipitates through a combination of thermodynamic calculations and experimental verifications. The effect of Al/Ti/Nb ratio on the phase relationship, nanoscale precipitation, and mechanical properties of the (CoCrFeNi)94-x-y-zAlxTiyNbz HEAs were systematically studied. At high Al/Ti ratio and low Nb addition, L12 precipitates are uniformly distributed in grain interiors, whereas L21 precipitates are formed mainly along grain boundaries. The decreasing Al/Ti ratio leads to the transition from the L12/L21 to L12-Eta co-precipitation structure. With the further increasing Nb to 2 at%, the L12/Laves configurations appear in the alloys with high Al/Ti ratios, while the L12/Laves/Eta co-precipitation is obtained at low Al/Ti ratio. It is noteworthy that the alloy with 3% Al, 3% Ti, and 1% Nb has a uniform microstructure with a high density of L12 nanoparticles. Our analyses reveal that the combination of chemical driving force, precipitate/matrix lattice misfit, and interface energy determines the formation and microstructure of the alloys. In addition, the increasing Nb and Ti additions increase the L12 lattice constant through sublattice occupancy, which leads to the increasing FCC/L12 lattice misfit and the equilibrium shape transition from sphere to semicuboidal.|
Second, this research demonstrated the feasibility of controlling the discontinuous and continuous precipitation of L12-strengthened high-entropy alloys through nanoscale Nb segregation and partitioning. Control of discontinuous and continuous precipitation of L12ordered precipitates is crucial for tailoring microstructure and mechanical properties of coherent precipitation-strengthened high-entropy alloys (HEAs). Here, we show that the appropriate addition of Nb not only suppresses discontinuous L12 precipitation through grain boundary segregation but also promotes continuous L12 precipitation through nanoscale solute partitioning. Specifically, we explore the effects of Nb on the discontinuous and continuous precipitation microstructures, grain boundary segregation, and mechanical properties of (CoCrFeNi)94-xAl3Ti3Nbx (x = 0, 0.4, 0.8, 1.6, and 2.3 at.%) HEAs. Atom probe tomography reveals that Nb exhibits preferential segregation at grain boundaries of HEAs, which substantially inhibits the grain-boundary precipitation and migration due to the synergistic effects of grain-boundary energy reduction and solute-drag, thereby suppressing discontinuous L12 precipitation at the grain boundaries. Moreover, Nb partitions to the continuous L12 nanoparticles in grain interiors, which leads to a high supersaturation for continuous L12 precipitation. Because of these beneficial effects, Nb-modified HEAs with a uniform distribution of L12 nanoparticles throughout the matrix were developed, and the correlation between the precipitate microstructure and mechanical properties of these HEAs is discussed. Third, this research reported a ultrastrong yet ductile Fe-Co-Ni-Cr-Al-Ti HEAs through introducing a unique coherent nano-lamellar designing philosophy. For achieving ultrahigh strength and ductility combination through the synergistic strengthening mechanisms, a unique nano-lamellar architecture was tailored. Unlike the traditional nano-lamellar materials with ultrahigh strengths but suffering from low tensile ductility, the FCC/L12 dual phase coherent nano-lamellar shows that markedly enhanced tensile ductility can be achieved in coherent nano-lamellar alloys, which exhibit an unprecedented combination of over 2 GPa yield strength and 16% uniform tensile ductility. The ultrahigh strength originates mainly from the lamellar boundary strengthening, whereas the large ductility correlates to a progressive work-hardening mechanism regulated by the unique nano-lamellar architecture. The coherent lamellar boundaries facilitate the dislocation transmission, which eliminates the stress concentrations at the boundaries. Meanwhile, deformation-induced hierarchical stacking-fault networks and associated high-density Lomer-Cottrell locks enhance the work hardening response, leading to unusually large tensile ductilities. The extraordinary mechanical properties of the coherent nano-lamellar materials offer tremendous potential for structural applications in aerospace, automotive, and energy industries. In addition, the fundamental concept of lamellar architecture engineering can be applied to many other metallic materials, including new-generation superalloys, titanium alloys, and advanced steels, to achieve enhanced properties for specific applications.
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